Threading stainless into plastic

[GordMay]

Originally Posted by ebs001

I’m not a plumber but I know in electrical it is only approved to screw metal into plastic and not the other way around as you have stated.

Indeed.

Originally Posted by Boatyarddog

The issue with cracking plastic when screwing in a metal fitting is the threads, tapered, verses machine threads...

In Canada, it's the differing coefficient(s) of expansion.

[OS2Dude]

Why did you go to that much trouble? There are commercially available marine pipe sections that you could use, both metallic and non-metallic.

Going into the the thread type, not all are the same and you can not mix and match them with a secure result. If you insist on using your stainless fitting, make sure it has the same thread type as the thru-hull.

I would also not mix metals. (Stainless & Bronze, for example) That will cause issues down the road.

[goboatingnow]

Originally Posted by mvmojo

I hope you meant bronze rather than brass. Not a good idea to use brass below the waterline. Brass is made from copper and zinc. In sea water, stray currents (galvanic corrosion) will over time deplete the zinc from the brass and cause it to become brittle and eventually break.

CR or DZR brass is of course adequate

[Tupaia]

Originally Posted by nzmal

Hull fitting... shut off.. t section

If you have a powerful wash-down pump it may pull air through the head visa vis if the wash-down pump is open ended the head may pull air.

[nzmal]

Ok to clear up conversation points...
I had the T section made because it needs to fit in a confined space and my buddy is an engineer who made it for me.
The deck wash is a low pressure system that is rarely used for cleaning anchor well etc.
The nylon sea cock fitting is being replaced with an original metal fitting tomorrow.

Thanks for all the perspectives.

[Dalestr]

Nobody seems to really defend plastic thruhull fittings which is sad. Forespar Marelon seacocks as an example are very good and strong. From the Sail magazine online article on seacocks: “..., Marelon is a polymer composite (glass-reinforced Dupont zytel to be exact), i.e., much tougher stuff, which is why these fittings have long since been accepted by the ABYC and certified by the ISO as meeting or exceeding the required mechanical properties for use on boats.”
In the USA seacocks generally come with straight, not tapered pipe thread so pipe dope seems better than Teflon tape for sealing/locking attached fittings. A metal NPS threaded fitting attached should be fine and no electrolysis concern. Metal hulls largely demand their use. Not recommended in wood.

[Manateeman]

Boatbuilders do not often admit when they make mistakes. The worst mistake I made in building my own yacht was installing Forespar Marelon seacocks.
Pure junk. Forespar knew they had a problem and changed the design as report after report became public of frozen open valves and broken handles.
They refused to even deal with boatbuilders. I will never use any plastic seacock in any vessel ever and that includes the supported ball valves which pretend to be true seacocks.
Marine how to has an excellent web page on how to properly install Groco adaptor plates and seacocks. Groco... No plastic period...the opinion of a professional boatbuilder with in depth experience. Not interested in arguing.
It’s your boat...do what you wish.
Happy trails to you
Captain Mark and his Professional Boatbuilding manatee crew.